isolationist (n.)

1899 in reference to U.S. foreign policy, "one who advocates a policy of non-participation in foreign affairs" (earlier in reference to treatment of leprosy), from isolation + -ist. As an adjective from 1920. Isolationism is attested in a general sense by 1902; in a U.S. geopolitical sense by 1919 in reference to opposition to joining the League of Nations.